Tropospheric wet path-delay measurements

A dual-channel microwave radiometer measuring the sky brightness temperature at the frequencies 21.0 and 31.4 GHz, an infrared spectral hygrometer (IRSH) measuring the ratio of the radiation from the sun at the wavelengths 931 and 880 nm, and radiosondes have been used simultaneously to determine the excess path length due to water vapor (wet path delay) of radio waves propagating through the troposphere. By a least squares fit of the measured parameters from the microwave radiometer and the infrared spectral hygrometer, respectively, to the wet path delay calculated from the radiosonde profiles, the following root mean square (rms) differences of the wet path delay in the zenith direction were obtained: infrared spectral hygrometer-radiosondes, 1.1 cm; microwave radiometer-radiosondes, 0.7 cm; and 0.5 cm for a selected group of "good weather" data. The wet path delay was also calculated from surface meteorological measurements alone and the rms difference compared with corresponding radiosonde data was 2.0 cm in the zenith direction.